The invention more particularly relates to hydraulic systems for automatic braking of preselected braking pressures of swing devices found, for example, in excavators and cranes. Swing drives usually comprise a hydrostatic drive having a hydraulic pump and motor, and associated gearing and controls that direct the horizontal rotation of booms found on excavator and cranes.
Swing drive arrangements have utilized the control of fluid velocity or flow to the motor through a directional control valve. With velocity or flow control, the operator selects the direction and flow of fluid at system pressure.
Typically, flow control of the swing drive provide free swing or coasting of the boom on cranes. That is, in the absence of a command signal in the hydraulic system, the boom or the boom and load will coast to a stop due to frictional forces without excessive oscillation of the boom cable or the load.
Excavators are usually arranged with flow control to provide blocked center braking of the boom. That is, the boom or the boom and load will immediately decelerate to a stop in the absence of a command signal. In such use, return flow from the motor is relieved at the motor work port by a relief valve at a predetermined pressure setting. The blocked center braking allows rapid alignment of the boom and load and also provides for maintaining the boom stationary with the excavator operating on an inclined surface.
It is also desirable, under certain conditions of operation, to brake the swing drive at a preselected reduced pressure; i.e. a pressure setting below the relief valve pressure setting.
It has been found that some operators, who have had their initial training and experience on a free swing braking arrangement on cranes, express a preference for the free swing feature when confronted with the operation of an excavator provided with a blocked center braking arrangement. Conversely, some operators, who have had their initial training on an excavator with the blocked arrangement, express a preference for the blocked center arrangement when confronted with the operation of a crane with a free swing braking arrangement.
In view of the foregoing, it is an object of this invention to provide a hydraulic circuit arrangement for automatic braking at preselected pressures of swing drives wherein an operator may selectively choose, by means of a simple adjustment, a free swing braking arrangement, or reduced pressure braking anywhere between the free swing and blocked center braking arrangements.
Accordingly, there is disclosed herein a selective swing drive automatic braking arrangement for a velocity control hydraulic system.
In accordance with the invention, the velocity control braking arrangement disclosed herein comprises a hydraulic control valve system, such as disclosed in U.S. Pat. No. 4,201,052 having a common assignee with the present application, including a pilot controller, a pump, and a hydraulic actuator. The actuator includes a movable element and a pair of openings adapted to function alternately as inlets or outlets for moving the element in opposite directions. The pilot controller supplies fluid to the system at pilot pressure and the pump supplies fluid at pump pressure to the motor. The control system includes a line adapted for connection to each of the openings. A meter-in valve means controls fluid flow from the pump to the motor and is selectively operable by pilot pressure from the pilot controller. A meter-out valve is associated with each of the lines for controlling fluid flow from the motor. The meter-out valves are each selectively pilot operated by pilot pressure from the pilot controller. In accordance with the invention, the supply fluid being supplied to the actuator is applied, at a predetermined pressure, to the meter-out valve means controlling flow from the actuator in opposition to the pilot pressure which tends to open the meter-out valve means.